1. Field of the Invention
The present invention relates to a control technique based on video signals. More specifically, the present invention relates to a control signal generation circuit, a video display device, and a control signal generation method for controlling luminance by executing signal processing according to video signals inputted from outside.
2. Description of the Related Art
Recently, regarding the power consumed in thin-type display devices, low power consumption is becoming advanced by employing LED for backlight (B/L), etc. However, the proportion of the power consumption of the backlight in the total power consumption is still great in such thin-type display devices. Therefore, for the liquid crystal in which the backlight is used by being lighted at all times, employed is a technique which controls the luminance of the backlight (B/L luminance) according to the inputted video signals to lower the power consumption as the whole device.
Further, also known is a structure in which the above-described luminance control technique is applied to an RGBW-type liquid crystal display device (also referred to as RGBW product hereinafter) in which a single pixel is constituted with four sub-pixels acquired by adding W (White) to sub-pixels of R (Red), G (Green), and B (Blue) (hereinafter, each of the sub-pixels R, G, B, and W is simply expressed as R, G, B, and W).
That is, there is also a technique with which the power consumption of the backlight is decreased by constituting an RGBW-type display device through combining the technique regarding the RGBW-type liquid crystal display panel in which W is added to R, G, and B in order to improve the luminance with the technique for controlling the luminance of the backlight and by allotting the luminance amount improved by adding W to the luminance decreased amount of the backlight.
Here, the panel characteristic of the RGBW-type liquid crystal display device will be described in a specific manner.
When all-white display is performed in the RGBW-type liquid crystal display panel exhibiting the panel characteristic with which the ratio of the maximum white luminance of W and the maximum white luminance generated by RGB becomes 1:1, it is simply considered that the luminance thereof becomes twice that of the case using RGB only.
However, when comparing it with the same size and the same resolution of an RGB product (a display panel in which a single pixel is constituted with three sub-pixels of R, G, and
B having no W sub-pixel), the area occupied by the other sub-pixels is decreased in the RGB product for the amount of the added W so that the area per sub-pixel is decreased to about ¾ since the structure that constitutes a single pixel with the three sub-pixels of R, G, and B is changed to the structure that constitutes a single pixel with the four sub-pixels of R, G, B, and W.
Further, in practice, there is also the area of the in-panel wiring and the like for driving the pixels, so that the area thereof per sub-pixel normally becomes smaller than ¾. Thus, strictly, it is necessary to think about it with the ratio of the numerical aperture areas where the light from the backlight can transmit through.
A strict value can be acquired by calculating the ratio of the numerical aperture of the sub-pixels of the RGB product and the numerical aperture area of the sub-pixels of the RGBW product as “Y” while assuming that the numerical aperture areas of the three sub-pixels constituting each pixel of the RGB product are the same and the numerical aperture areas of the four sub-pixels constituting each pixel of the RGBW product are the same. However, for convenience, the ratio is assumed as ¾ (=0.75) herein for explanation. The calculation method of “Y” and processing including those will be described later.
Assuming that the products are of the same size and the same resolution as described above, the ratio of the luminance of the RGBW with respect to the luminance of the RGB product in all-white display becomes 1.5 from “white of W+white of RGB=0.75+0.75”, and becomes 0.75 in primary-color display (R only, G only, or B only).
As described, when all-white display is done in the RGBW product, the luminance of 50% is increased with respect to that of the RGB product. When primary-color display is done, the luminance of 25% is decreased with respect to that of the RGB product.
Therefore, in a case of an image where the luminance is increased as in the all-white display, for example, the B/L luminance can be decreased to 66.6% from 100% that is the original luminance (1.5×0.666≈1) for the amount of the increased luminance 50%. That is, with the RGBW-type display panel employing the technique for controlling the B/L luminance, it is possible to decrease the power consumption of the backlight by about 33.3% while keeping the similar luminance as that of the original image in a case of all-white display.
The above is the base of the principle for driving the RGBW-type display panel that employs the technique for controlling the B/L luminance, which is devised to achieve low power consumption by reducing the power required for lighting up the backlight through decreasing the B/L luminance according to the video signals.
In the RGBW-type display device employing such technique, employed is a method which specifies the pixel to be the reference when determining the luminance decrease amount of the backlight and decreases the B/L luminance by corresponding to a feature value of the specified pixel.
That is, it is necessary to determine the value (feature value) to be a feature among the video signals of the image in order to determine the luminance decrease amount of the backlight. The feature value of the video signal is the value referred when changing the luminance of the backlight, so that it influences the image quality greatly.
There are various kinds of possible methods for determining the feature value and methods of luminance decreasing processing based thereupon. For example, known is a method which is designed to display an image similar to the original image while decreasing the B/L luminance by taking the pixel whose light-up amount of W in one frame of a video signal (W minimum light-up pixel) is the smallest as the reference, decreasing the B/L luminance as whole for the increase amount of the luminance of that pixel, and decreasing the luminance of the other pixels whose luminance is larger than the W minimum light-up pixel (the light-up amount of W of the other pixels is naturally larger since the W minimum light-up pixel is taken as the reference) by gradation conversion.
The W minimum light-up pixel taken as the reference in this case is the pixel whose saturation is the highest in one frame. For example, W is not lighted up (light-up amount of W is 0) in the pixel displaying the primary color (primary-color pixel). Thus, such pixel corresponds to the pixel whose saturation is the highest in one frame (W minimum light-up pixel). Further, in all-white display, there is only low saturation white. Thus, the pixel whose saturation is the highest in one frame corresponds to each white pixel.
As described, the technical content in which the pixel whose saturation is the highest in one frame is taken as the reference and the B/L luminance is decreased according to that is disclosed in Japanese Unexamined Patent Publication 2007-10753 (Patent Document 1), for example.
The RGBW-type display device of Patent Document 1 is designed to achieve low power consumption by the drive including backlight control (B/L control). This display device is structured to increase the gradation expansion rate as much as possible by gradation conversion of each pixel through executing the conversion with which the data allotted to white pixels (W) does not become the maximum but the data maximum values of each color become almost equivalent so as to increase the effect of decreasing the power consumption of the backlight by taking the data maximum value in one screen as the reference of the power consumption decrease amount of the backlight.
Further, regarding a way of increasing the effect of decreasing the power consumption of the backlight, there is also considered a method which calculates an average value of the saturation of the entire pixels (entire screen) of one frame and determines the decrease amount of the B/L luminance by taking the average value as the reference. With this method, in a case of a screen on which primary colors are displayed partially on a white screen where almost all-white (white is low in saturation) occupies one frame, the average value of the saturation in the entire screen becomes low. Thus, the luminance amplification rate becomes large, so that the decrease amount of the B/L luminance can be increased.
As other technical documents related to the RGBW-type display device, Patent Documents 2 to 4 in the followings are known, for example.
The display device disclosed in Japanese Unexamined Patent Publication 2008-131349 (Patent Document 2) employs a structure which prevents the colors (yellow and the like) to which W is added with high luminance from becoming too white by using a technique which expands the saturation (without expanding W-value) of only the data corresponding to RGB within the image data to be inputted and adjusts the W-value by the expanded luminance value. LUT (3DLUT) is used herein for conversion of the colors.
Japanese Unexamined Patent Publication 2009-210924 (Patent Document 3) discloses saturation/luminance decreasing processing for the RGB signals to be inputted as a technique used in a display device that employs B/L control. Specifically, it is a technique which decreases the backlight value by executing processing with which only the saturation is decreased in a case where a desired backlight value or less can be acquired by decreasing only the saturation while the saturation is decreased to “0” and the luminance is decreased in a case where the desired backlight value or less cannot be acquired by decreasing only the saturation.
Japanese Unexamined Patent Publication 2009-217052 (Patent Document 4) discloses an RGBW-type display device employing a technique which executes saturation decreasing processing on the inputted RGB signals and performs processing for decreasing, increasing, or not changing the luminance. The display device employing the B/L control employs a structure which adjusts the changing degree of the saturation and the luminance by designation of the luminance adjusting parameter in order to decrease the backlight value more securely.
However, in a case where the structure such as the technique disclosed in Patent Document 1 with which the pixel to be the reference of the power consumption decrease amount of the backlight is determined based on the maximum value of the saturation in one screen, the power consumption of the backlight cannot be decreased when there is even one pixel of high saturation and high gradation (primary colors, for example) in a screen whose saturation is medium saturation as a whole, for example.
That is, as described above, in a case where there is even one point of pixel whose light-up amount of W is “0” (pixel where W is not lighted up) existing in a screen when employing the way of decreasing the B/L control by taking the pixel whose light-up amount of
W is the smallest (W minimum light-up pixel) in one frame of a video signal as the reference, W for supplementing the luminance decrease amount of the backlight is not lighted up. Thus, the decreasing effect of the power consumption by the decrease of the B/L luminance may not be achieved.
Further, the method of determining the decrease amount of the B/L luminance by taking the average value of the saturation in one frame as the reference has such an issue that a sense of discomfort tends to be felt in the image quality.
For example, when a primary color is displayed partially in an all-white background, the average value of the saturation in the entire screen occupied by almost all-white (white is low in saturation) is an extremely small value. Thus, a sense of darkness and dullness in the color of the primary color display part is increased even though it is possible to increase the luminance decrease amount of the backlight. Therefore, a sense of discomfort in the image quality is felt with respect to the original image.
Such sense of darkness and dullness is caused when the light-up amount of W in the all-white display part is increased and the light-up amount of W in the primary-color display part becomes “0” (W is not lighted up).
That is, for an image in a state where the display part with high luminance is surrounding the periphery of the primary-color display part, human eyes recognize the luminance of the primary-color display part to be darker than the original luminance (simultaneous contrast effect). As a result, such sense of darkness and dullness felt in the color of the primary-color display part is increased.
As described above, in RGBW drive, a sense of discomfort is likely to be felt in the image quality when it is tried to increase the power consumption decreasing effect of the backlight. Meanwhile, the power consumption decreasing effect of the backlight is decreased when it is tried to suppress a sense of discomfort in the image quality.
Further, the technique of Patent Document 2 does not use the minimum value of RGB when calculating the W-value, so that the probability of lighting up W excessively is increased. Thus, such sense of discomfort in the image quality that it becomes whitish with respect to the original image is generated. In addition, the power consumption decreasing effect cannot be achieved since the B/L control is not performed.
The display device disclosed in Patent Document 3 executes a processing action for forcibly setting the saturation to be “0” as described above when the backlight value does not reach the prescribed decrease amount. Therefore, the hues of each pixel are changed greatly with respect to those of the original image, so that a sense of discomfort in the image quality is generated (issue of becoming whitish in particular).
The display device of Patent Document 4 performs processing for decreasing the saturation of the RGB signals in order to decrease the backlight value securely. Therefore, there is a possibility of causing a change in the hues of each pixel greatly with respect to those of the original image, so that a sense of discomfort may be generated in the image quality.
The display device of Japanese Unexamined Patent Publication 2009-47775 (Patent Document 5) performs processing for decreasing the saturation by disregarding the high gradation side of the RGB signals of inputted image data for decreasing the power of the backlight. Thus, the hues of each pixel are changed with respect to those of the original image, so that a sense of discomfort may be generated in the image quality.
The present invention is designed to improve the shortcomings of the related techniques described above. More specifically, it is an exemplary object of the present invention to provide a control signal generation circuit, a video display device, and a control signal generation method for minimizing a sense of discomfort as much as possible while decreasing the power consumption by luminance control of the backlight according to the video signals and maintaining the quality of the original image.